Grid monitoring problem,help please!

hell all, i set a point dpoint(x=0.5, y=0.0)at wall, the wall is ossilating with velocities=0.0 and displacements=0.0 initially, i monitor dpoint's y-coordinate to observe the track of the wall, but the curve puts 0.0088 at many initial time steps, it should be 0.0 at all events i think.